The ornamental design for a decorativebase for a fishbowl, as shown and described.
FIG. 1 is a front elevational view showing my new design, the rear
elevational view being identical; FIG. 2 is a top and right side
perspective view thereof; FIG. 3 is a top plan view thereof; FIG. 4 is a
right side elevational view thereof, the left side elevational view being
identical; and, FIG. 5 is a bottom plan view thereof. The broken lines in
FIG. 1 are for purposes of illustration only and do not form a part of the
claimed design.